1. Field of the Invention
The present invention relates to a camera filter unit, and particularly to a technique for improving a filter frame for holding an optical filter.
2. Description of the Related Art
The configuration shown in FIG. 5 is a known example of a camera filter unit mounted to a lens tube of a taking lens of a camera. This type of a camera filter unit is disclosed in JP-A 11-271840.
A camera filter unit 100 comprises a discoid optical filter 103, and a cylindrical filter frame 101 which holds the optical filter 103 on the internal side, as shown in FIG. 5. A male thread 101b is formed in the external peripheral surface of the filter frame 101 in an area of a specified width forward from the rear end edge. A female thread 101a is formed in the internal peripheral surface of the filter frame 101 in an area of a specified width rearward from the front end edge.
The male thread 101b in the external peripheral surface is used in order to mount the camera filter unit 100 to a lens tube 106, and the female thread 101a in the internal peripheral surface is used in order to mount a lens cap to the camera filter unit 100 or to join another camera filter unit to the front side. The optical filter 103 is fixed to a filter holder 102 formed in the internal peripheral surface of the filter frame 101.
To describe in greater detail, the filter holder 102 is formed to the rear of the female thread 101a in the internal peripheral surface of the filter frame 101, and the filter holder comprises a circular internal peripheral surface 102a of a specified width and an annular protuberance 102b which protrudes inward from the rear end edge of the circular internal peripheral surface 102a. The optical filter 103 is inserted into the filter holder 102 from the front side of the filter frame 101, an external peripheral surface portion 103a of the optical filter 103 is in contact with the circular internal peripheral surface 102a, and a rear end edge potion 103b of the optical filter 103 is in contact with the front end surface of the protuberance 102b. 
The optical filter 103 is pushed from the front against the protuberance 102b by an annular retaining ring 104, and is positioned in the direction of an axis line 100a. The retaining ring 104 has formed in the external peripheral surface a male thread 104a capable of threading with the female thread 101a, and the retaining ring 104 is threaded up to the end at the rear of the female thread 101a. A rear end surface 104b of the retaining ring 104 is in contact with a front end edge 103c of the optical filter 103.
The camera filter unit is usually mounted to the front end portion of the lens tube of the taking lens. Therefore, when the filter frame holding the optical filter is long in terms of its longitudinal dimension, there is a problem in that the front end portion of the filter frame gets in the angle of field of the taking lens. Therefore, a demand exists for a reduced dimension of the filter frame in a camera filter unit.
In a conventional camera filter unit 100, a dimension L of the filter frame 101 is a combination of the thickness dimension M of the optical filter 103, the width dimension N of the retaining ring 104 in the longitudinal direction, a first dimension O from the retaining ring 104 to the front end of the filter frame 101, and a second dimension P from the rear surface of the optical filter 103 to the rear end of the filter frame 101.
The first dimension O cannot be reduced because the area of a specified width from the front end edge of the filter frame 101 is a mounting area for mounting the lens cap or another component. The second dimension P cannot be reduced because a predetermined dimension must be ensured so that when the camera filter unit 100 is mounted to the lens tube 106 of the taking lens 105 of the camera, the front end surface 105a of the foremost lens constituting the taking lens 105 and the rear surface of the optical filter 103 do not interfere with each other. There is also a limit to reducing the width dimension N of the retaining ring 104 because in order to reliably fix the optical filter 103 to the filter frame 101, a width dimension must be ensured in the area over which the male thread 104a is formed in the external peripheral surface of the retaining ring 104. Furthermore, the dimension M is determined by the thickness dimension of the optical filter 103 being held. Therefore, there is a problem in that it is difficult to reduce the width L of the conventional filter frame 101 (to reduce the dimension L in the direction of the center axis line).